Process for improving a photosensitivity of an electrophotographic element

ABSTRACT

A PROCESS FOR IMPROVING THE PHOTOSENSITIVITY OF AN ELECTROPHOGRAPHIC ELEMENT WHICH IS COMPRISED OF CONDUCTIVE BASE, AN ORGANIC PHOTOCONDUCTIVE LAYER, AND AN ORGANIC SENSITIZER LAYER. THE PROCESS COMPRISES EXPOSING THE ELEMENT TO MORE THAN 10 LUX SECONDS OF VISIBLE LIGHT AND UNIFORMLY CHARGING THE ELEMENT TO A NEGATIVE POTENTIAL OF 200 TO 1800 VOLTS PRIOR TO CHARGING THE ELEMENT WITH A POSITIVE CHARGE AND THEN EXPOSING IT TO AN IMAGE. THE CHARGING TO A NEGATIVE POTENTIAL CAN BE CARRIED OUT PRIOR TO, SIMULTANEOUSLY WITH, OR SUBSEQUENT TO THE STEP OF EXPOSING THE ELEMENT TO THE VISIBLE LIGHT.

MITSUSUKE IKEDA ETAL, PROCESS FOR IMPROVING A PHOTOSENSITIVITY May 28, 1974 OF AN ELECTROPHOTOGRAPHIC ELEMENT Filed Oct.

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United States Patent 3,813,242 PROCESS FOR IMPROVING A PHOTOSENSITIVITY OF AN ELECTROPHOTOGRAPHIC ELEMENT Mitsusuke Ikeda, Hirakata, Hisanao Sato, lbaragi, Etsuko Torii, Kyoto, Kazuhisa Morimoto, Settsu, and Y0 Hasegawa, Suita, Japan, assignors to Matsushita Electric Industrial Co., Ltd., Kadoma, Osaka, Japan Filed Oct. 1, 1971, Ser. No. 185,660 Claims priority, application Japan, Oct. 7, 1970, 45/88,434; Oct. 13, 1970, 45/90,.354, 45/90,355

Int. Cl. G03g 13/22 U.S. Cl. 96-1 R 3 Claims ABSTRACT OF THE DISCLOSURE A process for improving the photosensitivity of an electrophotographic element which is comprised of conductive base, an organic photoconductive layer, and an organic sensitizer layer. The process comprises exposing the element to more than lux seconds of visible light and uniformly charging the element to a negative potential of 200 to 1800 volts prior to charging the element with a positive charge and then exposing it to an image. The charging to a negative potential can be carried out prior to, simultaneously with, or subsequent to the step of exposing the element to the visible light.

This invention relates to a process for improving the photosensitivity of an electrophotographic element comprising organic materials.

In electrophotography, an electrostatic latent image is formed by exposing a uniformly charged photoconductive layer to light which has passed through, or is reflected from, an original document. Said electrostatic latent image is developed by using a toner directly on the photoconductive layer or after the latent image has been transferred to a dielectric layer, or is changed to a frost image by heating.

Selenium and zinc oxide are well known as inorganic photoconductive materials useful in electrophotography. Recently, much attention has been paid to organic photoconductive materials because of their excellent characteristics such as transparency, light weight, flexibility, and flat surface. However, the electrophotographic sensitivity of organic photoconductive materials is usually much lower than that of an evaporated selenium photoconductive layer. An organic conventional electrophotographic element having sensitizers distributed uniformly therein has a considerable residue potential in the photo-decay process.

Such a disadvantage can be overcome by the electrophotographic element and the improved electrophotograpic element disclosed in our copending applications entitled Electrophotographic Element and Improved Electrophotographic Element, both filed concurrently here with Application. The electrophotographic element includes a conductive support and an organic photoconductive layer comprising a polymer and an organic sensitizer, said organic sensitizer being distributed locally at the top layer of said organic photoconductive layer. The improved electrophotographic element includes a conductive support, an organic insulating layer on said conductive support, and an organic layer contiguous to said sensitizing organic insulating layer and comprising an organic sensitizer. For convenience, this electrophotographic element will be called a two layer electrophotographic element. The two layer electrophotographic element has less residue potential but still is not entirely satisfactory with respect to the photosensitivity.

Therefore, a principal object of this invention is to provide a process for improving a photosensitivity of a two layer electrophotographic element.

3,813,242 Patented May 28, 1974 This object is achieved by a method of, prior to positively charging the element preparatory to exposing it to an image, exposing the element to a more than 10 lux seconds of visible light and uniformly charging the element to a negative potential of 200 to 1800 volts. The charging to a negative potential can be carried out prior to, simultaneously with, or subsequent to the step of exposing the element to the visible light.

Details of the process according to this invention will become apparent upon consideration of the following description taken together with the accompanying drawings in which:

FIG. 1 is a section through a two layer electrophotographic element.

FIG. 2 is a graph of sensitivity curves for a conventional electrophotographic element and a two layer electrophotographic element in both a conventional electrophotographic reproduction process and a novel process including the improved photosensitivity process of this invention.

Before proceeding. with a detailed description of the novel process contemplated by the present invention, the aforesaid two layer electrophotographic element will be described with reference to FIG. 1. A conductive support 1 supports an organic photoconductive layer 2 which is composed of a top layer 3 comprising an organic sensitizer and of a plain organic photoconductive layer 4 having no sensitizer therein. Said plain organic photoconductive layer 4 has a thickness of 3 to 30 microns and may comprise any organic material which is capable of transporting photo-carriers therethrough. Said top layer 4 has a thickness of 0.1 to 1 micron and may consist of only an organic sensitizer to produce photo-carriers upon illumination by light or may consist of an organic insulating material and an organic sensitizer to produce photocarriers upon illumination by light, Said organic insulating material may be diiferent from or the same as that of said plain organic photoconductive layer 4.

Said plain organic photoconductive layer comprises preferably a polymer selected from the group consisting of poly-N-vinylcarbazole, poly-N-alkenylcarbazole, and nucleo-substituted poly-N-vinylcarbazole. If necessary, said polymer can be combined with a plasticizer and polycarbonate in a suitable amount for improvement of mechanical properties.

Said polymer may be used for said organic insulating material having an organic sensitizer dispersed therein. Besides said polymer, any organic polymer having high electrical resistivity can be used for said organic insulating material. Among possible organic polymers, polystyrene and polycarbonate produce a preferred top layer.

Any sensitizer can be used and the more preferred are pyrylium salts described in U.S. Pat. 3,250,615, thiapyrylium salts described in U.S. Pat. 3,250,615, carbonium salts described in U.S. Pat. 3,575,698, benzopyrylium salts described in U.S. Pat. 3,526,502, benzopyrylium salt derivatives described in U.S. Pat. Application 4,200 filed Jan. 20, 1970, and mixtures thereof.

The two layer electrophotographic element can be prepared by any suitable and available method. For example, a solution including poly-N-vinylcarbazole, a plasticizer such as chlorinated diphenyl, or epoxy resin and polycarbonate in a solvent such as toluene, chlorobenzene, or dioxane is applied to a conductive support such as aluminum plate by a conventional coating method such as blade coating method and is dried to form said plain organic photoconductive layer. Another solution including N-vinylcarbazole, a benzopyrylium salt derivative as a sensitizer, a plasticizer such as a chlorinated diphenyl, and polycarbonate in a solvent such as alcohol or ketone is applied to the plain organic photoconductive layer by any suitable coating method such as whirler or kiss coating method and is cured to form said top layer.

A conventional process for forming an electrostatic latent image on the two layer electrophotographic element comprises a process of charging the two layer electrophotographic element to a positive potential and exposing it to a light image. Said latent image is developed by using a toner directly on the surface of said element or after the latent image is transferred to a suitable dielectric layer, or is changed to a frost image by heating.

It has been discovered according to the present invention that the two layer electrophotographic element can be given an improved photosensitivity when treated by an additional process which comprises charging the two layer electrophotographic element to a negative potential and uniformly exposing the two layer electrophotographic element to a visible light in advance of the conventional process which comprises charging the two layer electrophotographic element to a positive potential and exposing the positively charged element to a light image. The additional process makes the photosensitivity several times as produced by a single use of the conventional process.

The exposure eifective for improving the sensitivity of the two layer electrophotographic element is an exposure of more than 10 lux seconds, preferably more than 100 lux seconds of visible light by any suitable light source such as a tungsten lamp, a halogen lamp, a fluorescent lamp, a xenon lamp, or a high-pressure mercury lamp. Charging is, in general, by corona discharge means. The surface potential elfective for improving the sensitivity of the element is a negative potential of 200 to 1800 volts, preferably more than 500 volts.

There are many variations in the additional process for improving the sensitivity of the element described above. They are as follows:

Additional Process Variation 1 comprises exposing the two layer electrophotographic element to more than 100 lux seconds of visible light and subsequently charging said exposed element to a negative potential of 200 to 1800 volts.

Additional Process Variation 2 comprises exposing the two layer electrophotographic element to a negative potential of 200 to 1800 volts and simultaneously exposing said two layer electrophotographic element to more than 10 lux seconds of visible light.

Additional Process Variation 3 comprises charging the two layer electrophotographic element to a negative potential of 200 to 1800 volts and subsequently exposing said negatively charged element to more than 10 lux seconds of visible light.

The sensitivity becomes greater the higher the number of the additional process variation, that is, the sensitivity as a result of the additional process 3 is greater than that as a result of additional process 2.

Photosensitivity of the element is defined as a 10 percent-decay exposure, and said exposure is the exposure to reduce the surface potential of the element to 10 percent of the surface potential in the dark. The smaller the 10 percent-decay exposure, the more photosensitive the element.

Various processes according to this invention will be shown in the following examples.

EXAMPLE 1 An electrophotographic element is prepared. 1.0 gram of poly-N-vinylcarbazole (Luvican M-l70, available from BASF Company) and 0.5 gram of chlorinated diphenyl (Kanechlor #400, available from Kanegafuchi Chemical Industrial Company) are dissolved in 10 milliliters of toluene to form a solution. The solution is applied to an aluminum plate by blade coating and dried at 60" C. for 5 minutes to obtain a plain organic photoconductive layer of about microns in thickness. An acetone solution containing 1.0 gram of N-vinylcarbazole, 0.5 gram of chlorinated diphenyl and 0.1 gram of the reaction product of 2-(p-methoxystyryl)-3-phenylbenzopyrylium perchlorate and 2-(p-methoxystyryl)-3-phenyl- 4-methoxypyrane is applied to the plain organic photoconductive layer and heated at C. for 30 minutes to form a top layer of about 0.5 micron in thickness.

The thus obtained electrophotographic element is charged to a positive potential of about 800 volts by corona discharge means and exposed to a tungsten lamp (2800 K.). The decay curve of the surface potential of the element is measured by an electrometer. In this conventional process, the 10 percent-decay exposure of the element is 30 lux seconds.

Various additional process variation according to this invention for improving the sensitivity of the element are applied to this element. A tungsten lamp is used as a light source. After application of said process variations, the element is charged to a positive potential of about 800 volts and then exposed to a light. The 10 percent-decay exposure of these treated elements is shown in Table I. It is evident that the additional process in each of its variations bring about a substantial improvement in the sensitivity of the two layer element.

An improved electrophotographic element is prepared. A plain organic photoconductive layer of about 10 microns in thickness is formed on an aluminum plate in the same manner as the plain organic photoconductive layer in Example 1. A methylethyl ketone solution containing 1.0 gram of polystyrene (Dialex HF-55, available from Mitsubishi Chemical Company), 0.5 gram of chlorinated diphenyl and 0.1 gram of the reaction product of 2-(pmethoxystyryl)-3-phenylbenzopyrylium perchlorate and 2-(p-methoxystyryl)-3-phenyl 4 methoxypyrane is applied to the plain organic photoconductive layer and is heated at 100 C. for 30 minutes to form a top layer of about 0.5 micron in thickness. The thus obtained two layer electrophotographic element is first charged to 'a positive potential of about 800 volts and exposed to a tungsten lamp. The decay curve of the surface potential of the element is measured by an electrometer and is shown at B in FIG. 2. As shown in FIG. 2, this two layer element has a photoresponse when charged to a positive potential (curve B) but has no photoresponse when charged to a negative potential (curve D). The 10 percent-decay exposure of this element in the conventional electrophotographic process (curve B) is 30 lux seconds. The addition of the novel process comprising charging the element to a negative potential of about 1000 volts and exposing said charged element to an exposure of 500 lux seconds makes the photo-decay of the surface potential of the element in the conventional process more rapid (curve A) and the 10 percent-decay exposure becomes 15 lux seconds.

For comparison, a conventional electrophotographic element is prepared. A solution containing 1.0 gram of poly- N-vinylcarbazole, 0.5 gram of chlorinated diphenyl, and 5 milligram of the sensitizer the same as above in 10 milliliters of toluene is applied to an aluminum plate and dried at 60 C. for 5 minutes to form an organic photoconductive layer of about 10 microns in thickness. The amount of the sensitizer contained in said uniform photoconductive layer is the same as that in the top layer of the above electrophotographic element. The thus obtained conventional electrophotographic element is then charged to a positive or a negative potential of about 800 volts by corona discharge means and exposed to a tungsten lamp.

The measured decay curve of the surface potential of the element is shown at C in FIG. 2. This conventional element has a photoresponse when charged to a positive and a negative potential (curve C). The percent-decay exposure of this conventional element is about 100 lux seconds and can not be enhanced by the heating of the element or by the addition of the process of this invention for improving the sensitivity of the element.

In FIG. 2, the surface potential of the element is indicated on the vertical axis in volts, while exposure is indicated along the horizontal axis in lux seconds.

EXAMPLE 3 TABLE 11 Exposure (in lux seconds) after negative charging (1000 volts):

10 percent-decay exposure (in lux seconds) Virgin 30 5 i 21 10 16 30 13 100 12 300 11 EXAMPLE 4 An element is first prepared as in Example 1. The element is charged to a negative potential of about 1000 volts and subjected subsequently to an exposure of 500 lux seconds of a tungsten lamp. Some time after, the previously charged and exposed element is charged to a positive potential of about 800 volts and exposed to a tungsten lamp. As shown in Table HI, the enhanced high sensitivity of the element continues for to 60 minutes.

TABLE III Time after the addition of the process of this invention 10 percent-decay (in minutes): exposure 0 11 15 12 30 13 60 16 (24 hours) 30 EXAMPLE 5 An element is prepared as in Example 1. The element is first charged to a negative potential of 200 to 1800 volts by corona and then subjected to an exposure of about 500 lux seconds of a tungsten lamp. The charged and exposed element is then charged to a positive potential and exposed to a tungsten lamp. The 10 percent-decay exposure is shown in Table IV. It is evident that the previously charged and exposed element has excellent sensitivity.

6 TABLE IV Negative charging (in volt) 10 percent-decay What is claimed is:

1. In a process for forming a latent image on a two layer electrophotographic element having a conductive support supporting an organic photoconductive layer which is composed of a thin top layer comprising an organic sensitizer and a plain organic photoconductive layer having no sensitizers therein, said element being charged to ya positive potential, and the positively charged element being then exposed to a light image, the improvement comprising carrying out, prior to the step of charging to a positive potential, the steps of exposing said two layer electrophotographic element to more than lux seconds of visible light and subsequently charging said exposed element to a negative potential of 200 to 1800 volts.

2. In a process for forming a latent image on a two layer electrophotographic element having a conductive support supporting an organic photoconductive layer which is composed of a thin top layer comprising an organic sensitizer and a plain organic photoconductive layer having no sensitizers therein, said element being charged to a positive potential, and the positively charged element being then exposed to a light image, the improve ment comprising carrying out, prior to the step of charging to a positive potential, the steps of charging said two layer electrophotographic element to a negative potential of 200 to 1800 volts and simultaneously exposing said two layer electrophotographic element to more than 10 lux seconds of visible light.

3. In a process for forming a latent image on a two layer electrophotographic element having a conductive support supporting an organic photoconductive layer which is composed of a thin top layer comprising an organic sensitizer and a plain organic photoconductive layer having no sensitizers therein, said element being charged to a positive potential, and the positively charged element being then exposed to a light image, the improvement comprising carrying out, prior to the step of charging to a positive potential, the steps of charging said two layer electrophotographic element to a negative potential of 200 to 1800 volts, and subsequently exposing said negatively charged element to more than 10 lux seconds of visible light.

References Cited UNITED STATES PATENTS 2,987,395 6/1961 Jarvis 96l.7 3,355,289 11/1967 Hall et al 96--1.5 3,591,374 7/1971 Seus 961.6

CHARLES E. VAN HORN, Primary Examiner US. Cl. X.R. 96-1.6, 1.5 

